As measures to increase the storage capacity of an optical disc, there have been known use of laser beams having a shorter wavelength for recording/reproduction of data, reduction of the size of laser beam spots by the use of objective lenses having a higher numerical aperture, use of multilayer discs having plural recording layers, and others. As for the multilayer disc, two-layered discs have already been put into practical use for the DVDs and Blu-ray discs (hereafter, referred to as “BD”).
Recently, optical discs having three or more recording layers have been proposed, and specifically, BDs having six recording layers have been proposed (see, for example, ODS2006 Technical digest (2006)041).